Electrodeposition has become an important method for the application of coatings over the last two decades and continues to grow in popularity because of its efficiency, uniformity and environmental acceptance. Cathodic electrodeposition has become dominant in areas where highly corrosion-resistant coatings are required, such as in primers for automobile bodies and parts. Epoxy based systems provide the best overall performance in this application and are widely used.
Cathodic electrodeposition resins based on conventional epoxies obtained by reacting liquid diglycidyl ethers of bisphenol A with bisphenol A to produce higher molecular weight epoxy resins have known disadvantages. Such products tend to have excessively high softening points resulting in poor flow out. In addition, such products require excessive amounts of solvent during their preparation. Thus, Bosso et al., U.S. Pat. No. 3,839,252, describes modification with polypropylene glycol. Marchetti et al., U.S. Pat. No. 3,947,339, teaches modification with polyester- diols or polytetramethylene glycols. Wismer et al., U.S. Pat. No. 4,419,467, describes still another modification with diols derived from cyclic polyols reacted with ethylene oxide. These various modifications, however, also have disadvantages. Tertiary amines or strong bases are required to effect the reaction between the primary alcohols and the epoxy groups involved. Furthermore, these reactions require long reaction times and are subject to gellaton because of competitive polymerization of the epoxy groups by the base catalyst. In addition epoxy resins containing low levels of chlorine are required to prevent deactivation of this catalyst.
Copending Application Serial No. 887,849, now U.S. Pat. No. 4,698,141, filed July 18, 1986 by Anderson & Hickner: co-pending application Ser. No. 887,850, now U.S. Pat. No. 4,829,104 filed July 18, 1986 by McIntyre, Rao & Hickner: co-pending application Ser. No. 911,281, now abandoned filed Sept. 24, 1986 by Mclntyre: copending application Ser. No. 69,459 filed July 2, 1987; and co-pending application Ser. No. 069,475 filed July 2, 1987 describe the combination of aliphatic epoxy resins, such as the polyether diepoxides with aromatic epoxides for use in cathodic electrodeposited coatings. The resulting coatings are thicker than similar coatings not containing the aliphatic epoxides. It was discovered that these coatings were less viscous than the conventional coatings. Partial capping with monophenolic compounds reduced the average molecular weight of these resins which results in a lower viscosity resin. These resins are especially useful in the lower temperature cured coatings because they allow the deposited particles to coalesce and flow at the lower temperatures without requiring additional quantities of coalescent solvents. The use of these resins substantially reduce or may eliminate the need for the volatile organic coalescent solvents. U.S. Pat. No. 4,605,690 to Debroy et al describes the use of a soft, blocked isocyanate crosslinking agent as a reactive diluent in a cathodic electrodeposition bath which reduces the viscosity of the organic coating thus reducing or eliminating the use of coalescent solvents. Many of the coalescent solvents such as alcohols and glycols exemplified by U.S. Pat. No. 4,605,690 (col. 11, lines 9-18) would react with unblocked isocyanates thereby reblocking them, and thus substantially reducing the final crosslinked density.
Anderson et al. in U.S. Pat. Nos. 4,575,523 and 4,575,529, discloses a film-forming resin composition which when combined with a crosslinking agent and solubilized, is capable of depositing high build coatings in cathodic electrodeposition processes, with a low crosslinking temperature. Oxime blocked polyisocyanates may be used in the coatings. Anderson et al, in U.S. Pat. No. 4,596,744 teach the use of an oxime blocked isocyanate crosslinker in which the oxime blocking agents are the reaction products of acyclic aliphatic carbonyl-containing compounds having at least seven carbon atoms and a hydroxyl amine. The use of coupling solvents such as glycols and alcohols are taught in these three patents to obtain good emulsions and smooth deposited films.
Numerous U.S. Patents teach the use of blocked isocyanates as crosslinkers for film forming compositions in the cathodic electrodeposition process. In addition to the previously cited applications of Anderson et al., U.S. Pat. Nos. 3,894,922; 3,947,339; 3,984,299; 3,959,106; 4,017,438; 4,038,232; 4,031,050, 4,101,486; 4,134,816; 4,260,697; 4,297,255; 4,310,646; 4,393,179; 4,176,221; 4,182,831; 4,182,833; 4,225,478; 4,225,479; 4,339,369; 4,452,681; 4,452,930; and 4,452,963 disclose various blocked isocyanate crosslinkers which will deblock at temperatures below 160.degree. C., such as oxime blockers. These are useful in cathodic electrodeposition formulations. Isocyanate reactive solvents are generally taught as useful in these formulations.
U.S. Pat. Nos. 3,935,087; 3,947,338; 4,007,154; 4,009,133; 4,040,924; 4,081,343; 4,134,865; and 4,134,866 teach the use of partially blocked isocyanate crosslinkers which will deblock at temperatures below 160.degree. C. and which are reacted with base cationic resins that are useful in cathodic electrodeposition formulations. Isocyanate reactive solvents are generally taught as useful in these formulations.
The following U.S. Patents are low temperature cured cathodic electrodeposited coatings which do not cure via oxime blocked isocyanates. U.S. Pat. No. 4,435,559 teaches the use of beta-hydroxy urethane crosslinkers which are appreciably cured at 121.degree. C. and produce excellent cures at 163.degree. C. U.S. Pat. No. 4,440,612 discloses a coating comprising a polymeric polyol with a polyester crosslinking agent having at least two beta-alkoxyester groups per molecule which cures from about 150.degree. C. to about 205.degree. C. in 10 to 45 minutes. European Patent Application 0,125,577 and U.S. Pat. No. 4,610,769 describe the use of aminoplast curable cationic electrodeposition coatings which cure as low as 93.degree. C. U.S. Pat. Nos. 4,576,980 and 4,612,098 disclose a low temperature cured cathodic electrodeposition coating which employs azetidinedione compounds as crosslinkers. These patents teach the use of various additives such as alcohols, glycols, etc. which may be used as coalescent and coupling solvents to improve the film appearance.
Blocked isocyanate can be defined as an isocyanate reaction product which is stable at room temperature but dissociates to regenerate isocyanate functionality under the influence of heat.
Many coating formulations applied by electrodeposition include pigments to provide color, opacity, application, or film properties. U.S. Pat. No. 3,936,405, Sturni et al., describes pigment grinding vehicles especially useful in preparing stable, aqueous pigment dispersions for water-dispersible coating systems, particularly for application by electrodeposition. The final electrodepositable compositions, as described, contain the pigment dispersion and an ammonium or amine salt group solubilized cationic electrodepositable epoxy-containing vehicle resin and other ingredients typically used in electrodepositable compositions.
The present invention employs a blocked urethane crosslinker which, when blended with a cathodic epoxy resin and non-reactive solvents, and dispersed in an aqueous solvent, can be used for cathodically electrodeposited coatings which are cured by baking at temperatures below 160.degree. C. There is a need in the electrodeposition coating industry for a lower temperature cured, corrosion resistant coating. It has been determined that the described formulation produces a corrosion resistant, low temperature cured, cathodic electrodeposited coating. Not only would lower cure temperatures decrease the energy required to cure the coatings but it would allow preassembly of plastic components which normally distort due to the heat employed during curing of the coating. This could result in a savings in assembly costs.